The present invention relates to an improved process of the production of selectively hydrogenated polymers of conjugated dienes and more particularly to such a process utilizing a titanium hydrogenation catalyst.
U.S. Pat. No. 5,291,990 describes a process for the hydrogenation of conjugated diolefin polymers which first involves the polymerization of copolymerization of such monomers with an organo alkali metal polymerization initiator in a suitable solvent thereby creating a living polymer. The living polymer is terminated by the addition of hydrogen. Finally, selective hydrogenation of the unsaturated double bonds in the conjugated diolefin units of the terminated polymer is carried out in the presence of at least one bis(cyclopentadienyl)titanium compound preferably of the formula: ##STR1## wherein R.sub.1 and R.sub.2 are the same or different and are selected from the groups consisting of halogens, C.sub.1 -C.sub.8 alkyl and alkoxyls, C.sub.6 -C.sub.8 aryloxys, aralkyls, cycloalkyls, silyls and carbonyls. The hydrogenation step is carried out in the absence of hydrocarbon lithium and alkoxy lithium compounds.
We have found that the above process, while being extremely advantageous for the hydrogenation of most polymers, has some disadvantages. We have discovered that for low molecular weight polymers, i.e. molecular weights (true peak molecular weight determined by gel permeation chromatography) below about 100,000 and/or where the polymer concentration in solution is high, the above process may produce an excess of lithium hydride which appears to be detrimental to the successful hydrogenation because an excess of alkali metal, usually lithium, hydride (LiH) may aid in the ultimate destabilization of the hydrogenation catalyst. This problem is most evident when low levels of bis(cyclopentadienyl)titanium compounds are used. This problem could also occur when higher molecular weight polymers are hydrogenated when such polymers are in solution in higher than the normal concentrations (i.e. more than about 15 to about 35%).